In a wide variety of manufacturing and steel processing applications, it is often desirable or necessary to join together sheets or strips of steel, alloys or the like, such as by welding along a contiguous edge. Such joining can be accomplished by conventional seam welding equipment, butt welding equipment, arc welding apparatus, high energy lasers, electron beam or plasma arc welding devices.
It has been observed that the quality of the seam-weld joining sheets of material directly influences both mechanical and microstructural properties of the resulting assembly. Consequently, it is absolutely essential to optimize the quality of the weld in many of today's advanced technology applications. In particular, the quality of the weld effects the resulting material microstructure, microhardness, tensile properties, formability, fatigue strength, and fracture toughness, which all directly affect the overall value of the joining process and the resultant joined pieces. In short, the mechanical properties of the weld should be as close as possible to those of the base metals being welded. It should be noted that sheets to be joined need not be made of identical materials, and often are not. The registration or "fit up" of the opposing proximal edges of sheets of material to be joined is also critical to ensuring the optimal weld seam, especially in high speed laser-welding applications where a very narrow gap must be maintained along the seam due to the small focused laser beam spot size. A uniform weld profile is largely determined by the fit-up and gap width between the proximal edges when the weld is made.
Butt welding of steel strips and the like has been accomplished by the implementation of various arrangements such as that described in U.S. Pat. No. 4,623,777, which issued to M. Aihara et al. on Nov. 18, 1986. In particular, the Aihara reference describes the use of an inlet clamp and an outlet clamp for holding opposing edges of the steel members to be welded in butted position. One clamp is moveable relative to the other clamp to adjust the gap between the butted edges of the steel strips. As can be appreciated from a review of this reference, however, mechanical clamping of strips to be joined is relatively cumbersome and inefficient for use with larger sheets to be joined. In high speed applications, mechanical clamping structures can also interfere with required movement of the sheets and/or the welding device. The Aihara reference further emphasizes the problems generally encountered with non-uniform confronting edges of parts to be joined and the varying gap widths which can result from conventional raked shears and similar trimming devices.
U.S. Pat. No. 4,765,532, which issued to A. Uomoti et al. on Aug. 23, 1988, also incorporates mechanical clamping means for clamping the opposing end portions of successive strips to be joined. The Uomoti device further includes corrective machining means attached in conjunction with a moveable table to simultaneously machine the opposing edges of both strips in order to minimize the variance of gap therebetween when the table is moved toward a stationary table to butt the respective strips against one another. As in Aihara et al., however, the mechanical clamping structures of Uomoti are relatively large and intrusive, and tend to make the joining process inefficient as a result.
Recognizing fit-up or registration problems common in laser butt welding applications, U.S. Pat. No. 4,577,088, which issued to C. Sharp on Mar. 18, 1986, teaches that the opposed sides of the workpiece are to be canted in butting relationship in order to locate burrs on the edges away from the weld line. The workpiece in Sharp is contemplated as being a single piece formed into a cylindrical member and seam welded to form a can body or the like. The edges of the workpieces are urged toward each other during welding by the application of tangential forces applied through the mechanical clamping means which are located about the periphery of the workpiece. The Sharp method for laser butt welding is limited to forming can bodies or similar cylindrical objects, and includes the inherent deficiencies of utilizing cumbersome mechanical clamping devices. Moreover, the canted orientation of the workpiece is not broadly applicable to the much more common substantially planar relationship often required in butt joint procedures.
U.S. Pat. No. 4,733,815, which issued to J. Sturm on Mar. 29, 1988, discloses a process for guiding sheets on different conveying planes which are inclined at an acute angle to one another to enable continuous feeding of sheets to be butt welded using a stationary welding device. While an arrangement as contemplated in Sturm may be applicable in applications where continuous feeding of material is possible, such an arrangement is not easily adaptable to butt welding applications where smaller pieces are to be seam welded together in a non-continuous manner. Moreover, Sturm requires the use of large gripping rollers to be arranged above and below the strips or sheets adjacent the welding device. Similarly, the z-bar guide apparatus set forth in U.S. Pat. No. 4,354,090 (which issued to C. Nilsen on Oct. 12, 1982) provides oppositely disposed channels which guide the opposing edges of sheets together as they are continuously fed toward the discharge end of the z-bar. The channels are tapered and converge toward one another to provide progressively abutting edges of sheets as they move longitudinally through the z-bar arrangement.
Magnetic devices have been applied to arc welding applications such as shown in U.S. Pat. Nos. 4,436,980 and 4,443,686 which issued to N. Pache et al., and U.S. Pat. No. 4,714,818 which issued to K. Mazac; particularly for controlling the welding arc about the periphery of a weld seam to be applied to abutting pipe sections and the like. Magnets have also been utilized to provide hold-down forces to magnetic sheets to be welded together. In particular, U.S. Pat. No. 3,582,609, which issued to J. Morely et al. on June 1, 1971, describes base-mounted electromagnetic clamps on either side of the frame for gripping the adjacent ends of strips to be joined. Because the magnetic clamp of the Morely reference utilizes a magnetic bed plate below the material to be clamped and an electromagnet above, the actual material to be clamped need not be magnetic in nature. However, the Morely magnetic clamps are spaced from the welding zone to prevent changes in inductance in the welding circuit, and must utilize mechanical nose clamp devices to prevent buckling of the strip between the clamp and the welding zone. As a consequence, the clamping arrangement of Morely is quite cumbersome and restrictive.
U.S Pat. No. 3,701,881, which issued to W. Rother et al. on Oct. 31, 1972, contemplates the use of electromagnetic holding plates comprising electromagnetic coils and iron cores which are embedded in a non-magnetic material. The parts of the electromagnetic coils which are directed toward the upper side of the holding plates are covered by non-magnetic material to prevent interference with the welding arc. While these magnetic hold-down devices are clearly less cumbersome than the mechanical clamping devices widely utilized in the industry, heretofore there has not been available a simple device which can provide automatic registration and alignment of confronting edges of two or more plates to be butt welded together which does not require the use of complicated mechanical structures, continuous feeding of the sheets themselves, or other cumbersome moving parts to ensure the intimate contact along the abutting edges of the sheets to be joined.